Researchers have long agreed that it is more appropriate to view child and adolescent psychopathology as multi-dimensional than un-dimensional. Consequently, the field has a long tradition of attempting to use empirical methods to define the multiple dimensions of psychopathology. In spite of these efforts, however, there is still marked disagreement about the number and nature of dimensions of psychopathology in youths. This disagreement on the fundamental units of measurement seriously limits scientific progress in the study of etiology. The proposed study will use a combination of psychometric and behavior genetic methods to resolve disagreements about the basic dimensions of psychopathology in children and adolescents. Using data from a sample of twins, confirmatory factor analysis will be used to test the fit of alternative methods of the underlying structure of child and adolescent psychopathology derived from prior research and theory. The proposed study will go well beyond factor analysis, however, by validating the putative dimensions using both criterion validity and behavior genetic strategies. Two putative dimensions of psychopathology that do not show different patterns of association with external criterion variables would not be considered to be meaningfully distinct dimensions. Similarly, two putatively distinct dimensions that do not exhibit genetic and/or environmental influences would not be considered to be distinct dimensions. Potential development and gender differences in the dimensions of psychopathology will be evaluated using structural equation modeling.